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Green Building Bible, Fourth Edition
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Housing - New Build: Urgent help needed with floor construction detail.

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  1.  
    • CommentAuthorPigglet
    • CommentTimeMay 8th 2014
     
    I'm in the middle of replacing an old conservatory with what I hope will be a much more efficient garden room.
    I'm currently (as in today) in the process of constructing the floor but have just had a phone call from the builder that has halted me in my tracks!

    Proposed construction is slate tiles on a reinforced 125mm thick concrete slab with the UFH pipes cable tied to the rebar in the middle of the slab. Slab will sit on 150mm of Celotex which sits on a dolomite base, so

    20mm slate flooring
    125mm reinforced slab with UFH in centre and 25mm celotex around perimeter.
    150mm celotex
    200mm compacted dolomite.

    The builder has just enquired about the concrete and when he told the supplier what it was for the supplier said we should be using no more than a 50mm screed on top of the UFH pipes.

    My logic with our initially proposed method of construction is that once up to temperature it should provide a relatively stable heat output and allow us to add heat to it whenever we want to as opposed to just when we want that room warm.

    It would also act as a bit of a heat/cool store in summer helping to regulate the temperature of the room if it picks up solar gain during bright days?????

    Can anyone advise on which approach may be best as I want to get the floor finished asap but don't want to have to dig up the 13tonnes of dolomite I'm about to put down
    :shamed:
  2.  
    Agree: 50mm on top of heating. As long as there is no insulation on top of or just underneath the heating, the whole slab will still act as a heat sink.
    You do have a DPM somewhere?
    • CommentAuthorPigglet
    • CommentTimeMay 8th 2014
     
    Sorry I don't know what you're agreeing with archess, to clarify do you agree with my method of 125mm slab acting as thermal mass or agree that there should only be a 50mm screed on top of the UFH heating pipes meaning little thermal mass?
    • CommentAuthortony
    • CommentTimeMay 8th 2014
     
    it will be fine as proposed, I would like to see more insulation but then I always do.
    • CommentAuthormw116
    • CommentTimeMay 8th 2014
     
    Agree with Tony, and with your intention for the thermal mass you'll get as a result, the 125mm slab is right. True, 50mm screed is the norm, but that doesn't make it 'wrong' to do as you are doing.
    • CommentAuthorPigglet
    • CommentTimeMay 8th 2014
     
    Great, thanks for the reply's folks.
    I would of liked more insulation in the slab, I intended to put 200mm in but everyone I spoke to (builders, architect and insulation suppliers) said I was going way over th top and may have compression issues. Withthat in mind I spoke to an insulation supplier about Dow Floormate XPS and they told me it was not worth the significant cost increase over celotex in a domestic situation hence me going with 150mm of celotex. If it will make that much difference I may be able to squeeze another 50mm in?????
    • CommentAuthortony
    • CommentTimeMay 8th 2014
     
    I would go 200 eps, it won't squash, I been doing it for many years without problems, we do compact the base nice and flat first. Use better insulation round the edges.
    • CommentAuthorEd Davies
    • CommentTimeMay 8th 2014
     
    200 mm Springvale EPS70, λ = 0.038 W/(m·K), U = 0.19 W/(m²·K)
    150 mm Celotex, λ = 0.022 W/(m·K), U = 0.146 W/(m²·K)

    Wondering why tony is suggesting less insulation :shocked: ? Is the EPS enough cheaper to worry about or is it the compressive strength of concern here?

    Even EPS200 (if that's what “200 eps” means, rather than 200 mm of EPS) has λ = 0.033 W/(m·K) so 200 mm is still more conductive than 150 mm of Celotex. About breaks even if you're a bit cynical about the long term insulation of Celotex and assume it drops to λ = 0.025 W/(m·K) eventually - both about 0.166 W/(m²·K).
    • CommentAuthortony
    • CommentTimeMay 8th 2014 edited
     
    I add in the insulating effect of the ground, don't trust the short or long term stats on pir then there is the question of the foil.

    And I don't like the high price of the more expensive choice. I generally used 2 x 100mm of eps.
    • CommentAuthorPigglet
    • CommentTimeMay 9th 2014
     
    I already have the celotex so going with EPS isn't really an option now. I put most of the dolomite in late yesterday and have compacted it down really tight in 100mm layers using a big diesel forward and reverse wacker.
    I think I may be able to get another 50mm of insulation in place of the last layer of dolomite so will have a measure up when it stops raining.

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